Generally, amplitudes of signal lights are attenuated by losses on an optical-fiber transmission-line in the case of optical fiber transmission communication between two points. The attenuation in the amplitudes of signal lights is one of factors for deterioration in the signal light quality at a receiving terminal. A transmission method for realizing transmission between two points under compensation for losses in signal lights in order to prevent deterioration in signal quality caused by attenuation in the amplitudes of signal lights is called an optical amplification transmission method.
As compared with the optical amplification transmission method, a transmission method for realizing transmission between arbitrary two points without recovery of attenuated amplitudes of signal lights in the middle of transmission is called as a nonrepeartered transmission method. The nonrepeartered transmission method has an advantage that a transmission system may be established at more reduced cost than that of the optical amplification transmission method, as there is nonrepeartered transmission in the middle of transmission.
Now, the optical-fiber nonrepeartered transmission method will be described, referring to FIG. 7 to FIG. 11. Here, FIG. 7 is a view which shows a relation between the span loss and the received-signal to noise ratio (“received SNR”) in optical-fiber nonrepeartered transmission. FIG. 8 is an explanatory view of Raman gain-bandwidth generated by a Raman pump light. FIG. 9A and FIG. 9B are explanatory views of Raman amplification. FIG. 10 is a configuration example of a nonrepeartered transmission system where the Raman amplification effect is applied. FIG. 11 is an explanatory view of Raman amplification effect in the system shown in FIG. 10.
In FIG. 7, the vertical axis shows received SNR (dB), and the resolution is @ 0.1 nm. The horizontal axis shows span loss (dB). The longest transmission distance in the nonrepeartered transmission system largely depends on the losses on the transmission line and the transmission input power. Generally, obtaining a received SNR of 15.6 dB (resolution: @0.1 nm) is a criterion for accurate receiving of signal lights in the case of signal lights with a signal transmission speed of 10 Gbit/second.
That is, a received SNR of 15.6 dB or more is obtained in the case of characteristics (1) where the input transmitting power is 0 dBm/ch, when the transmission-line loss is about 37.5 dB or less. Thereby, it is shown that nonrepeartered transmission over about 150 km may be realized, when the loss on an optical-fiber transmission-line is assumed to be 0.25 dB/Km. On the other hand, the transmission-line loss becomes 42.5 dB or less in order to obtain a received SNR of 15.6 dB or more, and, accordingly, transmission over about 170 km may be realized in the case of characteristics (2) where the input transmitting power is increased by 5 dB. In a word, the transmission distance may be made longer according to the increased amount of the input power. Moreover, when the transmission-line loss is not 0.25 dB/km, but 0.2 dB/km, transmission over a distance of 187.5 km may be realized at an input power of 0 dBm/ch, and transmission over that of 212.5 km may be done so at an input power of 5 dBm/ch.
As described above, a method for increasing the input power and a method for using optical fibers with small losses per unit length are considered to be used as a method for making the transmission distance longer in the nonrepeartered transmission. However, the increase in the input power is limited by the influence of nonlinear effects of optical fibers, and there is also a limitation in reduction in losses of the optical fibers. Therefore, application of the Raman amplification using the Raman amplification effect of the optical fibers to a method for making the distance longer in the nonrepeartered transmission has been noticed.
In the Raman amplification effect, optical fibers themselves on which signal lights are transmitted are used as an amplification medium, and crystal lattice vibration of the material forming the optical fibers is caused by pump lights launched into the optical fibers. At this time, induced amplification of scattered lights, called Stokes lights, is performed by the interaction with the optical phonons caused by the crystal lattice vibration to a frequency shifted to a shorter frequency by a certain proper amount than frequency of pump light. The amplification gain caused by the Raman amplification effect depends on the material of the optical fibers, and, generally, has a Raman gain-bandwidth shown in FIG. 8. That is, the horizontal axis shows wavelength (nm), and the vertical axis shows Raman gain coefficients in FIG. 8. As shown in this figure, a wavelength at which the maximum gain is obtained is a wavelength 100 nm-110 nm away from the pump wavelength. The gain-bandwidth lies in a wavelength range over about 60 nm in a long skirt extending over the side of the shorter wavelength from the center wavelength causing the maximum gain.
A method (called as forward pump) for entry of pump lights in the same direction to the propagating one of the signal lights, and a method (called as backward pump) for entry of pump lights in the opposite direction to the propagating one of the signal lights are used as a method for entry of the pump lights for the Raman amplification. FIG. 9A and FIG. 9B each show a relation between the power of the signal light (Signal Power (dBm)) and the power of the pump light (Pump power (W)) to the distance (Distance) when the Raman amplification has been performed on an optical fiber with a length of 100 km. FIG. 9A is for the forward pump, and FIG. 9B is for the backward pump. In FIGS. 9A, 9B, the characteristics (a) show power characteristics of the signal lights, and the characteristic values can be read at the left-side vertical axis (signal light power (Signal Power (dBm)) shown by an arrow pointing to the left side. And, the characteristics (b) show power characteristics of the pump lights, and the characteristic values can be read at the right-side vertical axis (pump light power (Pump Power (W)) shown by an arrow pointing to the right side.
As the Raman amplification effect depends on the power of the pump lights, and the power of the pump lights are attenuated by the fiber losses, the Raman amplification gain is gradually reduced along with increase in the propagation distance. Accordingly, in the case of the forward pump scheme, the signal lights are amplified as the power of the pump lights is large near at the entry terminal, and the signal lights are decreased as the signal lights approach the emitting terminal (receiving terminal) as shown in FIG. 9A. Conversely, in the case of the backward pump scheme, the Raman amplification gain is almost zero as the power of the pump lights is small near at the entry terminal of the signal lights, and the signal lights are amplified due to the large power of the pump lights as the signal lights approach the emitting terminal (receiving terminal) as shown in FIG. 9B. Moreover, it has been generally known in the case of comparison between the forward pump scheme and the backward pump scheme that the amount of crosstalks caused by influence of the pump lights on the signal lights is more advantageously smaller for the backward pump scheme.
Now, a uni-directional and nonrepeartered transmission system where Raman amplification by backward pump is applied will be described, referring to FIG. 10. as shown in FIG. 10, a transmitter 91 is provided at one terminal of an optical-fiber transmission-line 90, and a wavelength-selection-type optical combining and branching filter 92 is provided at the other terminal of the optical-fiber transmission-line 90. A receiver 94 is connected to the optical combining and branching filter 92 through an optical isolator 93, and a Raman pump light source 95 is also connected to the filter 92.
A signal light S output from the transmitter 91 is entered from the one terminal of the optical-fiber transmission-line 90, and transmitted on the optical-fiber transmission-line 90 along the first transmission direction D1 toward the receiver 94. On the other hand, a Raman pump light P generated in the Raman pump light source 95 is entered into the optical-fiber transmission-line 90 from the other terminal through the optical combining and branching filter 92, and transmitted on the optical-fiber transmission-line 90 along the second transmission direction D2 toward the transmitter 91. The signal light S propagating on the optical-fiber transmission-line 90 in the first transmission direction D1 reaches the other terminal of the optical-fiber transmission-line 90, as the signal light S is gradually amplified by the Raman amplification effect of the Raman pump light P propagating in the second transmission direction D2, and is taken into the receiver 94 through the optical combining and branching filter 92, and the optical isolator 93.
The Raman amplification effect in the system shown in FIG. 10 will be described, referring to FIG. 11. FIG. 11 shows relations of the power of the signal lights (Signal Power) to the distance (Distance) of the optical-fiber transmission-line without the Raman amplification and with the Raman amplification (the power of the Raman pump lights: 25 dBm), when it is assumed that the signal transmission speed is 10 Gbit/second, and the transmission-line loss is 0.25 dB/km.
In FIG. 11, the characteristics (1) show the power characteristics without the Raman amplification, and the characteristics (2) indicate the power characteristics with the Raman amplification. In the case of no Raman amplification, the power of the signal lights linearly decrease according to increase in the transmission distance, as shown in the characteristics (1). It is shown that the point B of the characteristics (1) indicates the minimum receiving level (−37.5 dBm/ch), and the longest transmission distance realizing a received SNR of 15.6 dB or more is 150 km.
On the other hand, the signal lights are more amplified as the signal lights approach the receiving terminal in the case of Raman amplification by the backward pump, and accordingly, the power of the signal lights get out of the decreasing tendency, changes to the increasing tendency as the transmission distance increases, shows a receiving level of −32.5 dBm/ch at the point A corresponding to the point B, and further increases toward the receiving terminal, as shown in the characteristics (2). The difference of 5 dB between the minimum receiving levels of the point A and point B is the Raman amplification effect. In a word, it is shown that the longest transmission distance realizing a received SNR of 15.6 dB or more is 170 km in the case of the Raman amplification. As described above, the transmission distance may be made longer according to the difference in the minimum receiving levels under the Raman amplification, comparing with that of a case with no Raman amplification.
It is usual in optical communication systems to perform not only communication in the only one direction, but also communication in the opposite direction. A method for providing single-wire optical-fiber transmission-lines for each uni-directional communication, that is a method for providing two-wire optical-fiber transmission-line, and a single-wire bi-directional transmission method for realizing bi-directional optical communication on a single-wire optical-fiber transmission-line are used as a method for realizing bi-directional optical communication.
The single-wire bi-directional transmission method is excellent in system establishment at reduced cost, and soon, as efficiency in use of optical fibers maybe improved, and the number of optical fibers may be reduced. Moreover, as understood from the description, it may be that it is preferable to apply the Raman amplification in order to make the transmission distance longer when a nonrepeartered transmission system using a single-wire bi-directional transmission method is established. Even the forward pump method is basically acceptable, the backward pump method is recommended as an pump method, considering the transmission characteristics.
Following problems exist in the single-wire bi-directional transmission method where the Raman amplification by backward pump is applied. These problems will be explained while referring to FIG. 12. FIG. 12 shows an example of configuration in which single-wire and bi-directional transmission is realized by application of the Raman amplification based on backward pump in a nonrepeartered transmission system.
As shown in FIG. 12, a wavelength-selection type optical combining and branching filter 101 is connected to one input/output terminal of an optical-fiber transmission-line 100, and a wavelength-selection-type optical combining and branching filter 102 is connected to the other input/output terminal of the optical-fiber transmission-line 100.
An input/output terminal 103 and an optical isolator 104 are connected to the optical combining and branching filter 101. A transmitter and a receiver which are not shown are connected to the input/output terminal 103. A Raman pump light source 106 is connected to the optical isolator 104. An input/output terminal 107 and an optical isolator 108 are connected to the optical combining and branching filter 102. A transmitter and a receiver which are not shown are connected to the input/output terminal 107. A Raman pump light source 109 is connected to the optical isolator 108.
The first signal light S01 input to the input/output terminal 103 from the not-shown transmitter, and the first Raman pump light P01 generated in the Raman pump light source 106 are combined in the optical combining and branching filter 101, and are transmitted on the optical-fiber transmission-line 100 along the first transmission direction D1 toward the other input/output terminal. On the other hand, the second signal light S02 input to the input/output terminal 107 from the not-shown transmitter, and the second Raman pump light P02 generated in the Raman pump light source 109 are combined in the optical combining and branching filter 102, and are transmitted on the optical-fiber transmission-line 100 along the second transmission direction D2 toward one input/output terminal. The object of the first Raman pump light P01 is amplification of the second signal light S02, and the object of the second Raman pump light P02 is amplification of the first signal light S01. Moreover, it is assumed that the center wavelengths of the first signal light S01 and the second signal light S02 lie in Raman gain-bandwidths uniquely defined according to each pump wavelength of the first Raman pump light P01 and the second Raman pump light P02.
The Raman amplification effect for the first Raman pump light P01 over a short distance step on the optical-fiber transmission-line 100 is shown by the following equations (1) to (3). In these equations, S01: the first signal light, S02: the second signal light, P01: the first Raman pump light, P02: the second Raman pump light, gR: a Raman gain coefficient, ωs01: the center frequency of the first signal light, ωS02: the center frequency of the second signal light, ωP01 the center frequency of the first Raman pump light, ωP02: the center frequency of the second Raman pump light, αS01: a loss coefficient of the first signal light on an optical-fiber transmission-line, αS02: a dissipation coefficient of the second signal light on an optical-fiber transmission-line, and αP01: a dissipation coefficient of the first pump light on an optical-fiber transmission-line.
                                          ⅆ                          S              01                                            ⅆ            z                          =                                            g              R                        ⁢                          P              01                        ⁢                          S              01                                -                                    α              S01                        ⁢                          S              01                                                          (        1        )                                                      ⅆ                          S              02                                            ⅆ            z                          =                                            g              R                        ⁢                          P              01                        ⁢                          S              02                                -                                    α              S02                        ⁢                          S              02                                                          (        2        )                                                      ⅆ                          P              01                                            ⅆ            z                          =                                            -                                                ω                  P01                                                  ω                  S01                                                      ⁢                          g              R                        ⁢                          P              01                        ⁢                          S              01                                -                                                    ω                P01                                            ω                S02                                      ⁢                          g              R                        ⁢                          P              01                        ⁢                          S              02                                -                                    α              P01                        ⁢                          P              01                                                          (        3        )            
As understood from the equations (1) to (3), the Raman pump light is consumed in proportion to the product of the power strength of the signal light and the power strength of the pump light in the Raman amplification effect, when the signal light exists within the Raman gain-bandwidth uniquely defined by the wavelength of the Raman pump light. That is, there is much more consumption in the strength of the Raman pump light caused by a signal light with stronger power strength than that of a signal light with weaker power strength, when the signal light with strong power strength and the signal light with weak power strength are entered into the transmission line 100 at the same time.
Therefore, in FIG. 12, the power of the Raman pump light is greatly consumed by amplifying the first signal light S01 transmitted in the first transmission direction D1 with regard to the first Raman pump light P01 originally to amplify the second signal light S02, as the power strength of the first signal light S01 is stronger than that of the second signal light S02 in the vicinity of the output terminal of combined lights of the optical combining and branching filter 101, where the first Raman pump light P01 is injected into the optical-fiber transmission-line 100. Accordingly, the power of the first Raman pump light P01 for amplifying the second signal light S02 is lost, and it becomes impossible to obtain desired receiving characteristics for the second signal light S02.
Similarly, the power of the Raman pump light is greatly consumed by amplifying the second signal light S02 transmitted in the second transmission direction D2 with regard to the second Raman pump light P02 originally to amplify the first signal light S01, as the power strength of the second signal light S02 is stronger than that of the first signal light S01 in the vicinity of the output terminal of combined lights of the optical combining and branching filter 100, where the second Raman pump light P02 is injected into the optical-fiber transmission-line 100. Accordingly, the power of the second Raman pump light P02 for amplifying the first signal light S01 is lost, and it becomes impossible to obtain desired receiving characteristics for the first signal light S01.